Normally, keystone distortion occurs in an image projected by a projector on a screen due to the relative positional relationship between the projector and the screen. Further, non-linear distortion is sometimes present due to a local unevenness or a twist in a screen where projection is performed.
To correct such distortion, Japanese Laid-open Patent Publication No. 2010-028411 discloses a technique of capturing, by a digital camera or the like, a screen on which a pattern image for calibration formed from a specific shape array is projected, calculating the degree of distortion from a shift between an ideal position and the actual extraction position of a feature point extracted from a captured image based on the shape array, and projecting, from a projector, an image which has been corrected so as to eliminate the calculated distortion:
In the case of comparing feature points of an image in an ideal state and a distorted image, a method is generally applied of calculating the amount of distortion by assigning grid array information including a column component and a row component to each feature element and comparing the feature points assigned with the same array information.
Accordingly, in Japanese Laid-open Patent Publication No. 2010-028411 described above, with respect to feature elements in a projected image, the same grid array information is assigned to a feature point existing at a closest position to a feature element in an original image before projection.
The technique as described above is effective in a case where the amount of distortion is small, but in a case the distortion is great, for example, in a case the pedestal of a projector is greatly tilted, a feature point at a different position may be positioned closest to another feature point in the original image, and in this case, wrong array information is assigned to the feature point.